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Galactic Bubbles Spark Debate

New microwave and radio observations resurrect controversy over gigantic lobes seen ballooning from the Milky Way’s center.

The saga of the Milky Way’s bubble-blowing past gained a couple of twists over the holidays. For several years now, evidence has grown for a short-lived event a few million years ago that blasted two gargantuan lobes from the galaxy’s center. These bubbles, each roughly 25,000 light-years long, extend above and below our galaxy’s spiral disk.

Careful analysis of observations made by the Planck satellite produced this all-sky image at 30 and 44 GHz, which reveals a microwave haze ballooning outward from the Milky Way's center.

ESA / Planck Collaboration

Astronomers discovered the bubbles in gamma-rays in 2010, using the Fermi Gamma-ray Space Telescope. But that discovery wasn’t the first in the epic tale. Since 2004 astronomers have been debating a similar feature called the WMAP “haze,” a faint microwave fog that appears in some analyses of the WMAP satellite’s observations — but not in others. Notably, the WMAP team has been among the naysayers. When the Planck team announced their conclusive detection of microwave bubbles last year, and that these bubbles lined up with the Fermi ones, WMAP researchers stayed mum until they could complete their final analysis of all nine years of WMAP data.

Just before Christmas the team released the 9-year results. The main paper is 176 pages long and full of all sorts of goodies, but tucked in with the various cosmological results — which are WMAP’s well-deserved claim to fame — is a short discussion of the purported haze. The conclusion? Inconclusive. The astronomers find they can reproduce the haze detection, but they can also explain the data without it.

What makes the haze so complicated is how it’s teased out of the observations. Astronomers take all of WMAP’s data — which include signals from the cosmic microwave background and electrons corkscrewing around magnetic fields, among other things — and subtract maps of these different signals to peel apart the layers. The assumptions that go into this map-peeling process can influence the result. Lead author Charles Bennett (Johns Hopkins University) says that it’s possible that the fog signal was “soaked up” by other components the WMAP team peeled out. But he affirms that he and his colleagues see no unambiguous haze.

When superimposed over data from NASA's Fermi Gamma-ray Space Telescope, the galactic haze as seen by ESA's Planck mission lines up with the gamma-ray bubble features. The Planck data (red and yellow) correspond to emission at frequencies of 30 and 44 GHz, while the Fermi data (blue) correspond to observations performed at energies roughly a quadrillion times greater.

But Planck scientist Charles Lawrence (Jet Propulsion Laboratory) says Planck’s galactic map includes observations from seven frequency bands, giving scientists more information about the various signals mixed together in the microwave data. The Planck team also included WMAP’s 7-year observations in their analysis, bringing the total number of frequency bands to 12. So not only does Planck have a more complete picture of the microwave emission, its haze detection benefits from WMAP’s low frequency observations, Lawrence says.

Radio Lobes Detected

Soon after the WMAP 9-year results came out, an international team working with observations from the Parkes Radio Telescope in Australia announced in the January 3rd Nature its own detection of gigantic bubbles. The observations are part of the S-band Polarization All Sky Survey, or S-PASS, a project completed in 2010 that mapped polarized emission across the entire Southern Hemisphere sky.

Giant magnetized outflows (blue) extend several thousand light-years above and below the Milky Way's center in this composition based on radio and optical observations. The bubbles' curvature is real.

Polarized emission allows astronomers to map synchrotron radiation, the radio waves emitted by super-speedy electrons spiraling in a magnetic field. Those who favor the microwave haze say that it’s made of synchrotron radiation, and indeed, the two lobes Ettore Carretti (CSIRO Astronomy and Space Science, Australia) and his colleagues found are synchrotron. The lobes also roughly line up with the Fermi bubbles — an expected correlation, Carretti says, because the same electron population emits both the radio and gamma-ray photons.

The radio lobes appear to lie behind the Milky Way’s Sagittarius arm, suggesting they emanate from the galactic center, just like the Fermi bubbles.

What’s controversial about the Nature paper is the team’s theory for what created the bubbles. Astronomers haven’t agreed on an origin for the features, although lately the most popular explanation has been a violent fit by the Milky Way’s supermassive black hole. But Carretti’s team favors supernovae. Bursts of stellar death in star-forming regions planted around the galactic center could blast out the radio-emitting electrons that make up the bubble, explaining a few of the unique features they detect, the team says.

Polarization researcher Alan Kogut (NASA Goddard) says he’s not quite convinced. Carretti’s team notes that the radiation from the galactic center breaks a well-known relation between infrared and radio emission, seen both in our galaxy and in others. The infrared emission comes from heated dust, while the radio comes from synchrotron electrons. Both of these are kicked out by exploding stars, Kogut says. But there’s too much infrared emission in the galactic center, and the radio emission concentrates in the lobes — nothing like what supernovae should produce. On the other hand, an accreting black hole would preferentially jettison radio-emitting electrons, explaining the bubbles.

Carretti says that in fact the broken infrared-radio relation supports stellar culprits. Astronomers know that stars populate the galactic center, so there should be lots of radio along with the infrared — and there isn’t. But the amount of radio emission in the lobes matches what’s missing, Carretti says. Supernova outflows could have pushed the radio-emitting particles out of the galactic center, creating the lobes his team observes.

Either way, the bubbles now appear conclusively in both gamma-rays and radio, and — depending on whom you ask — in microwave. What we need to figure out now is whether the culprit was the black hole beast or its golden hoard.

About Camille M. Carlisle

Science Editor Camille M. Carlisle covers science news for Sky & Telescope. She specializes in black holes, galactic cosmology, and whatever she happens to be writing about at the time. She and Robert Naeye co-edited S&T's special issue Mars: Mysteries and Marvels of the Red Planet, published summer 2014.

Great question. The Milky Way is essentially moving to the left in the image, toward the Andromeda Galaxy. But astronomers don’t agree on why the lobes are skewed. Some have suggested a "wind" effect from the Milky Way’s passage through the intergalactic medium (the lateral motion you refer to); Carretti and his colleagues suggest instead that the bend happens because the outflows are wound up due to the galaxy’s rotation and the conservation of angular momentum. It’s really one more mystery in this enigmatic tale.